For example, if there is a code block that needs to be wrapped within another function call, it'll rewrite the entire function call and you'll just have to pray that the re-written code block wasn't subtly changed.
I _think_ so far it hasn't introduced any changes....
You can also unit test the function to better assure behavior didn’t change.
Still not an excuse to not read every line of course...
Unit tests give me the confidence that at least those tested logic paths are unaffected.
Sometimes with older codebases one cannot assume the paths have adequate test coverage.